1. Field of the Invention
The present invention relates to a deposited-film forming apparatus suitable for forming a deposited film of aluminum or the like on the surface of a work piece such as a rare earth metal-based permanent magnet.
2. Description of the Related Art
A rare earth metal-based permanent magnet such as an R—Fe—B based permanent magnet, of which an Nd—Fe—B based permanent magnet is representative, is used at present in a variety of fields, because it has a high magnetic characteristic.
However, the rare earth metal-based permanent magnet contains metal species (particularly, R) liable to be corroded by oxidation in the atmosphere. Therefore, when the rare earth metal-based permanent magnet is used without being subjected to a surface treatment, the corrosion of the magnet is advanced from its surface due to the influence of a small amount of acid, alkali and/or water to produce rust, thereby bringing about the deterioration and dispersion of the magnetic characteristic. Further, when the magnet having the rust produced therein is incorporated into a device such as a magnetic circuit, there is a possibility that the rust is scattered to pollute surrounding parts or components.
With the foregoing in view, it is a conventional practice to form a deposited film of aluminum or the like on the surface of a rare earth metal-based permanent magnet for the purpose of providing an excellent corrosion resistance to the rare earth metal-based permanent magnet.
Examples of conventionally known apparatus used for forming a deposited film of aluminum or the like on the surface of a rare earth metal-based permanent magnet, include an apparatus described in U.S. Pat. No. 4,116,161 and an apparatus described in Graham Legge “Ion Vapor Deposited Coatings for Improved Corrosion Protection” Reprinted from Industrial Heating, September, 135-140, 1994. FIG. 9 is a diagrammatic front view (a partially perspective view) of the inside of a vacuum-treating chamber 101 connected to an evacuating system (not shown) in one example of such apparatus. Two cylindrical barrels 105, for example, formed of a mesh net of a stainless steel are disposed side-by-side in an upper area in the chamber for rotation about a rotary shaft 106 on a horizontal rotational axis. A plurality of boats 102, which are evaporating sections for evaporating aluminum as a depositing material, are disposed on a boat support base 104 risen on a support table 103 in a lower area in the chamber.
With this apparatus, a plurality of rare earth metal-based permanent magnets 130 as work pieces are placed into each of the cylindrical barrels 105, and aluminum is evaporated from the boats 102 heated to a predetermined temperature by a heating means (not shown), while rotating the cylindrical barrels about the rotary shaft 106, as shown by an arrow in FIG. 9, thereby forming a deposited film of aluminum on the surface of each of the rare earth metal-based permanent magnets 130 in the cylindrical barrels 105.
The deposited-film forming apparatus shown in FIG. 9 is capable of treating a large amount of the work pieces and excellent in productivity. However, damage may be observed in some cases on the deposited film of aluminum formed on each of the rare earth metal-based permanent magnets. This damage exerts an adverse influence to the provision of a corrosion resistance to the rare earth metal-based permanent magnets, causing the increase in yield to be impeded. In addition, projections may be produced in some cases on the deposited film of aluminum formed on each of the rare earth metal-based permanent magnets, and when the magnet is incorporated into a part using an adhesive, such projections may exert an adverse influence to the adhesion.